Conventionally, lithium primary batteries have widely been used for appliances whose ambient temperature is approximately −20° C. to 60° C., which is close to the temperature range of human life. However, the application range of battery-powered appliances has recently been expanding, and the operating temperature range of such appliances also tends to expand commensurately. In in-car appliances, for example, there is an increasing demand for primary batteries that can maintain their functions for a certain period of time in a condition whose maximum ambient temperature can possibly become 125° C. and that are operable even at a low temperature of approximately −40° C.
However, since the positive electrode of a lithium primary battery contains manganese oxide, the catalytic action thereof decomposes a non-aqueous electrolyte in a high temperature range, which may cause an increase in the inner pressure of the battery. If the inner pressure of a coin battery increases, the contact among battery components is impaired, causing an increase in internal resistance. Also, in a cylindrical battery with a current-collecting structure employing leads, a rise in battery inner pressure may cause electrolyte leakage or the like, leading to degradation in battery characteristics. Particularly when a battery is exposed to a temperature environment of 100° C. or higher, the amount of gas evolved in the battery increases, possibly resulting in a large expansion of the battery. Such expansion significantly impairs the contact among battery components, thereby increasing the possibility of a rise in internal resistance, electrolyte leakage, and breakage.
Also, under a high-temperature environment, dissolution of manganese from the positive electrode is promoted, and the dissolved manganese is deposited on the negative electrode to form a high resistance film thereon, which may result in an increase in battery internal resistance. Particularly when a lithium primary battery is used over an extended period of time under a high-temperature environment, the dissolution of manganese from manganese oxide is promoted, so that the high resistance film deposited on the negative electrode surface becomes firm. The high resistance film significantly decreases the electrical characteristics of lithium primary batteries, and particularly decreases the large-current discharge characteristics and pulse discharge characteristics at low temperatures remarkably.
On the other hand, in the field of lithium secondary batteries, there has been a proposal to reduce the specific surface area of a spinel-type lithium manganese oxide used as a positive electrode active material, in order to limit the reaction site of the lithium manganese oxide and an electrolyte and therefore to suppress electrolyte decomposition and manganese dissolution (Japanese Laid-Open Patent Publication No. Hei 10-321227, Japanese Laid-Open Patent Publication No. 2002-117896, and Japanese Laid-Open Patent Publication No. 2003-346805). Likewise, in the field of primary batteries, it is considered that reducing the specific surface area of manganese dioxide also enables electrolyte decomposition and manganese dissolution to be suppressed to a certain degree.
However, manganese oxide used in a conventional primary battery is made of γ-type manganese oxide obtained by heat-treating β-type electrolytic manganese dioxide or the like at 350° C. to 430° C., or a mixed crystal of γ-type manganese oxide and β-type manganese oxide (hereinafter referred to as γ-β-type manganese oxide) (Japanese Laid-Open Patent Publication No. Sho 57-4064). With respect to the γ-β-type manganese oxide, its specific surface area can be reduced only to approximately 5 m2/g even if electrolytic conditions and baking conditions are changed. Therefore, there is a limit to suppressing electrolyte decomposition and manganese dissolution by reducing the specific surface area. Also, when the specific surface area of manganese oxide is reduced, the reaction site is limited, so there is a disadvantage that the low temperature characteristics (particularly low temperature discharge characteristics) are degraded. There is a trade-off between this disadvantage and the suppression of gas evolution and the like.
In the field of lithium secondary batteries, there has been a proposal to replace part of the manganese element of a spinel-type lithium manganese oxide with another element, such as magnesium, aluminum, iron, calcium, or chromium, in order to strengthen the structure of the lithium manganese oxide and therefore to suppress the dissolution of manganese into an electrolyte (Japanese Laid-Open Patent Publication No. 2000-327332).
In the field of lithium secondary batteries, there has been another proposal to use as a positive electrode active material a manganese oxide whose crystal structure is intermediate between the spinel-type lithium manganese oxide and the λ-type manganese oxide, in order to suppress the destruction of the crystal structure of the positive electrode active material. Such manganese oxide is prepared by removing part of lithium included in the spinel-type lithium manganese oxide by means of acid treatment (Japanese Laid-Open Patent Publication No. Hei 11-329424 and U.S. Pat. No. 4,312,930).
Further, regarding the conventional temperature range, not the high temperature range of 100° C. or more, there has been a proposal to use a lithium alloy in the negative electrode, in order to suppress the formation of a film that serves as a resistance component on the negative electrode surface, reduce the internal resistance of a primary battery, and improve its reliability (Japanese Examined Patent Publication Hei 7-63016).
As described above, in primary batteries, despite the attempts to reduce the specific surface area of manganese oxide for suppressing electrolyte decomposition and manganese dissolution in the positive electrode, the specific surface area can be reduced only to approximately 5 m2/g, and hence, its effects are limited. Also, when the specific surface area of manganese oxide is reduced, the reaction site is limited, so there is a disadvantage that low temperature characteristics are lowered. Also, when a lithium alloy is used in the negative electrode to reduce internal resistance, the film formation on the negative electrode surface can be suppressed, but it is not possible to suppress the emission of gas due to electrolyte decomposition and the dissolution of manganese element and the like from the positive electrode active material, which are causes of the film formation.